1. Field
This disclosure relates to a device and a method for measuring noise characteristics of an imaging sensor.
2. Description of the Related Art
Recent imaging sensors have an increased pixel value. This has resulted in a tendency for the S/N ratio to being degraded. Pixel value represents a feature quantity (output value) for each pixel of an imaging sensor and is contained in the imaging data. For example, the pixel value may indicate concentration, brightness, luminance, color difference, intensity, or the like.
The noise contained in the imaging data is caused by various factors, and the pixel value Po of the imaging data can be expressed by equation (1),Po=Pt+Pn   (1)In equation (1), Pt represents a true pixel value and Pn represents noise.
It is generally known that the distribution of noise Pn can be approximated as normal distribution. FIG. 7 shows the distribution of the pixel value Po. The pixel value Po is dispersed at a standard deviation σ from the true pixel value Pt due to noise Pn.
The standard deviation σ (or deviation or dispersion) of the noise Pn in equation (1) is dependent on the true pixel value Pt. As shown in FIG. 8, the standard deviation σ increases as the true pixel value Pt increases, and the noise Pn increases as the true pixel value Pt increases.
The noise characteristics of the imaging sensor used in a digital camera are generally measured in the following manner. An imaging subject such as a Macbeth chart 91, which includes a plurality of color samples as shown in FIG. 9, is first prepared. Each color sample must have a flat portion 92 (monotone portion that does not include changes). Then, the Macbeth chart 91 is imaged with a digital camera (or prototype) that incorporates the imaging sensor that is subject to the measurement. From the obtained imaging data, partial image data corresponding to each flat portion 92 of the color sample is retrieved. The average value and deviation value of pixel values is then calculated for the retrieved partial image data corresponding to each color sample. In this manner, the noise characteristics of the imaging sensor are obtained.
There is a necessity to prepare and image the imaging subject (Macbeth chart 91) that includes a plurality of color samples, each having the flat portion 92. This is burdensome in that many tasks are necessary.
Presently, there are no means for obtaining the noise characteristics that reflect the level of the pixel value from the imaging result of an arbitrary imaging subject without using a color sample. Further, the noise characteristics cannot be obtained at the same time as when imaging an arbitrary imaging subject.
Japanese Laid-Open Patent Publication No. 63-253481 proposes a scheme for measuring noise characteristics by determining the average value of pixel values and the dispersion of circuit pixel values. The publication also proposes a scheme for determining Gauss data noise dispersion or image noise dispersion.
Japanese Laid-Open Patent Publication No. 2003-85553 proposes a noise characteristic measurement circuit that divides a pixel region of an imaging sensor into a plurality of small regions and outputs the average value and dispersion of the pixel values for each small region. Japanese Laid-Open Patent Publication No. 2003-85553 proposes the use of various values generated from such average value and dispersion.